The present invention relates to a manufacturing method of optical films, optical films, and polarization plates and image display apparatuses employing these.
Optical films used in image display apparatuses are required to have durability towards changes in the environment in addition to having high performance and high quality. Large sizing has progressed recently in image display apparatuses, and improving these characteristics is being demanded more and more.
Particularly in liquid crystal display apparatuses, when said optical film is being used as the polarization plate protective film, it sometimes causes striations in the image display due to variations in the environment, particularly when that polarization plate protective film includes cellulose resin, it was desired to improve the dimensional stability of the film due to environmental variations.
When grains are present in a film, since the strength increases due to the dispersion of stress applied to the film or due to the presence of materials that are harder than the resin, as a consequence, it is expected to obtain an optical film having improved dimensional stability of the film with respect to environmental variations.
In order to improve slipping (anti-blocking) of cellulose resin film such as cellulose acetate film or to improve its resistance scratching, including or coating various types of grains has been known as is described in Japanese Unexamined Patent Application Publication No. Sho 62-37113, Japanese Unexamined Patent Application Publication No. Sho 61-94725, and Japanese Unexamined Patent Application Publication No. 2006-57109.
However, for improvement of dimensional stability of the film, if an attempt is made to compound the grains in the resin that forms the optical film, it is difficult to obtain the desired dispersion state in the film matrix, and very often the grains were agglomerating. When grains are added with the purpose of giving improvement in the film quality or improvement in the display performance, if the desired dispersion state is not obtained, it is not possible to realize the functions and effects expected from adding grains, but on the other hand the presence of grains may even have bad effect on the film quality. In particular, a big problem is the generation of haze or bright point impurities.
The following methods can be considered for compounding grains in a film.
In general, to begin with, a liquid with grains dispersed in it is prepared, and next, addition of the grain dispersed liquid is made either in the batch mode or in the inline mode during the process of manufacturing the optical film by the solution casting method.
In order to stabilize the dispersion of the grains, it is common to feed the dispersed liquid that has grains as a dispersed liquid (including emulsions and suspensions) in which the grains have been stabilized in the solvent by at least one of the dispersion stabilization methods of adding dispersing agents, surfactants, or carrying out surface treatment, etc., on the grains. In this case, since a dispersed liquid having grains is used, some times the grains in the dispersed liquid either got settled down or got agglomerated, and the grains could not be compounded uniformly in the resin and it was not possible to realize the desired functions. In particular, if an optical film is manufactured by the solution casting method using a dispersed liquid having agglomerated grains, very often bright points or introduction of foreign matter is seen that are caused by agglomerated grains in the obtained optical film.
Further, it is necessary to restrict the deadline for use of these dispersed liquids or to carry out control of the storage conditions, and became a factor causing variations in the quality during processing. Further, in a factory in which the solvent is being recovered in the solution casting method, bringing in dispersed liquid from the outside would increase the amount of solvent caused by the dispersed liquid, and sometimes there was a load on the solvent recovery capacity. In addition, in a factory that is recycling the solvent, since solvent is being brought in to the factory apart from that being recycled, it becomes necessary to strengthen the function of the solvent recovery equipment, or else, if the amount of accumulated solvent increases the upper limit, it is necessary to increase the storage tanks, or to dispose of the solvent that is brought in.
On the other hand, as a different method of adding grains to resin is that of adding the grains to the resin and kneading them. There is the method of powdering after mixing a solid material based on resin powder or resin pellets with the grains, and also, there is the method of mixing the solid material with the grains and then palletizing the melted resin material.
Incidentally, there may be considered the method of heating a solid material based on resin powder or resin pellets and taking it into the melted state and then mixing grains with it and kneading it, and the method of palletizing the melted material. However, in these methods, it may be considered that thermal dissociation takes place of at least a part of the mixed and kneaded material at the time of heating and melting, and it may be estimated that the molecular weight of the resin goes down, and the strength of the optical film prepared from this goes down becoming brittle, and the mechanical strength as an optical film sometimes become insufficient. In addition, it may be estimated that thermal dissociation of the mixed and kneaded material occur during heating and melting, it may get colored which is not desirable as an optical film, or noise may be generated. Therefore, in order to avoid thermal dissociation, it may be considered desirable to carryout mixing and kneading at as low a temperature as possible.
In general, while a plasticizer is added to the mixed and kneaded material for the purpose of controlling the moisture permeability of the optical film, as another effect of adding a plasticizer is that it is possible to heat and melt and knead at a lower temperature than the, melting point of the resin itself. However, the following points are estimated that when a film is prepared by solvent casting using these mixed and kneaded materials, because of additional heat being applied, the strength as an optical film decreases, the plasticizer may solve out at high temperature and high humidity, thereby becoming an impediment to reaching the quality required of an optical film.